1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly to a feedback air/fuel ratio control system for an internal combustion engine equiped with a secondary air supply passage in the air induction system.
2. Description of Background Information
The feedback air/fuel ratio control system is used in an internal combustion engine for the purpose of the emission control, and in which the air/fuel ratio of the mixture to be supplied to the engine is controlled to the stoichiometric value according to an output signal level of an oxygen sensor disposed in the exhaust system of the engine.
As a type of the above mentioned air/fuel ratio control system having an air induction side secondary air supply device, the present applicant has proposed a feedback air/fuel ratio control system in which the air flow through the secondary air supply passage communicated with the downstream of the throttle valve is controlled by an electro-magnetic valve which is open/close controlled in response to an output signal of the oxygen sensor, and the secondary air supply passage is provided with a vacuum responsive type air control valve for controlling the amount of the secondary air, which air control valve is applied with a control vacuum obtained by combining the vacuum of the vicinity or the downstream of the throttle valve and the so called venturi vacuum, i.e., the vacuum of the inside of the venturi.
On the other hand, in the case of the conventional air/fuel ratio control system, the system is designed to start the feedback control when a detected value of the engine coolant temperature has risen above a predetermined level. When the engine coolant temperature is below the predetermined level, the system is operated under an open loop control mode which provides an air/fuel ratio other than the stoichiometric air/fuel ratio. This is because during a period after a cold start of the engine in which the temperature of the engine coolant is low, the frictional resistance in the engine is relatively large and the atomization of the fuel is not sufficient and also the oxygen sensor remains inactivated during this period.
In addition, the air/fuel ratio of the mixture supplied to the engine cylinders is considered to be a function of the density of the intake air. Therefore, when the temperature of the intake air is low, the air/fuel ratio becomes leaner due to the density of the intake air which is greater than that in the period of normal engine operation. Therefore, during the open loop control mode after the cold engine starting, the air/fuel ratio of the mixture is shifted to the rich side so as to stabilize the engine operation.
However, in the case of the conventional system in which the start timing of the feedback control is determined in accordance with the engine coolant temperature, the drawback was that the start timing of the feedback control is sometimes improper because the intake air temperature may remain low even if the engine coolant temperature has reached the first predetermined level, or because it may take a long time to raise the engine coolant temperature to a second predetermined level higher than said first predetermined level.